The present invention relates to a color image display tube apparatus, and, in particular, to a color image tube apparatus which improves deformation of an electron beam shape and deformation of a grid image pattern by controlling a deflection magnetic field.
A conventional shadow mask in-line type color image display tube apparatus, as shown in FIG. 1, comprises a neck 2 having three electron guns 1 disposed therein horizontally in an in-line form, a face plate 6, which is coupled through a funnel 3 to the neck 2 and has a fluorescent screen 5 with fluorescent layers adhered to the inner surface thereof in such a specific manner that the layers provide three colors, red, green and blue, when hit upon by electron beams 4R, 4G and 4B from the respective electron guns 1, a shadow mask 7 disposed close to the face plate 6 and having a plurality of openings through which the electron beams 4R, 4G and 4B selectively hit on the associated fluorescent layers, and a deflection device 8, which is mounted on the outer wall of the funnel 3 and generates horizontal and vertical deflection fields to deflect the electron beams in horizontal and vertical directions, respectively. In this type of color image display tube, there are various methods known to converge each of the three electron beams on one point on the fluorescent screen.
One of the methods produces deflection fields called self-concentration type fields, which are horizontal and vertical deflection fields respectively formed in pincushion and barrel shapes. Accordingly, this method can provide good convergence of the electron beams nearly over the entire fluorescent screen without using other specific beam-converging means. According to this method, however, since the horizontal and vertical deflection fields are not uniformly formed, the electron beams would have a deflection-originated deformation, thus deforming a displayed image.
Another method, as disclosed in Japanese Patent Application Publication Nos. 53-1014 and 54-292273, causes the electron beams from the electron guns to be mutually parallel to one another and provides the three color image signals corresponding to the three electron beams with mutual delays, thereby converging the three electron beams. This process will be explained referring to FIGS. 2A and 2B.
FIG. 2A illustrates the positions on the fluorescent screen on which the electron beams hit with the image signals given with no time delays, using three vertical broken lines as one set for three colors. In this case, the three electron beams would have a mutual positional deviation of .DELTA.W on the fluorescent screen.
FIG. 2B illustrates the convergence of the three electron beams with the G and B image signals given with predetermined delays. More specifically, the G signal is given with a delay of .DELTA.W and the B signal is given with a delay of 2.DELTA.W to be in line with the R signal.
According to this method, the convergence is high and the horizontal and vertical deflection fields have more uniform shapes than the self-concentration type fields explained with regard to the first method, thus providing a little improvement to the deflection-originated deformation of the electron beams.
However, the image deformation caused by this deflection-originated deformation of the electron beams according to this method is still not negligible from a practical point of view.
FIG. 3A illustrates that an electron beam 9 horizontally deflected by the above conventional method is affected by the arrow direction influence by a horizontal deflection field 10. As a result, as shown in FIG. 3B, a spot displayed on the screen by the horizontally-deflected electron beam would have a horizontally-elongated bright core section 11 and a dark halo section 12 elongated in the vertical direction.
Similarly, FIG. 4A illustrates that an electron beam 13 vertically deflected is affected by the arrow direction influence by a vertical deflection field 14. As shown in FIG. 4B, a spot displayed on the screen by the vertically-deflected electron beam would have a vertically-elongated bright core section 15 and a dark halo section 16, which surrounds the core section 15 and is elongated in the vertical direction.
To eliminate the cross-sectional deformation of the electron beam, the horizontal and vertical field may further be shaped more uniformly. With this measure, however, the beam convergence would be deteriorated. Further, in this case, as shown in FIG. 5, since the amount of the positional deviations of the three electron beams B1, G1, R1 on the fluorescent screen differ particularly depending on the vertical deflection positions, the mutual delays between the image signals for attaining the convergence should be changed according to the vertical deflection positions. In this case, of a set of three electron beams R1, G1, B1, for example, the beam R1 that is positioned last in the horizontal scanning direction (i.e., on the right edge side of the screen) is used as a reference beam and the remaining beams B1 and G1 are given with delays. For example, at a horizontal scanning position D-D', the beams B1 and G1 are given with delays of 2.DELTA.W.sub.1 and .DELTA.W.sub.1, respectively, and at a horizontal scanning position E-E', the beams B1 and G1 are given with delays of 2.DELTA.W.sub.0 and .DELTA.W.sub.0 , respectively. This eliminates the mutual positional deviation of the three electron beams so as to improve the beam convergence.
Even with the use of this method, however, an image may still be deformed. FIG. 6 is given for explaining how the image deformation occurs; vertical lines of grid patterns appearing on the fluorescent screen would be slightly curved in a bow shape, as indicated by the solid lines with respect to reference grid images indicated by the broken lines. This means that a straight line-shaped object would be displayed on the screen in the curved or bow shape, and is crucial to image display tube apparatuses.
As explained above, although the conventional methods, when used to converge three electron beams, can improve the deformation of the electron beams, the improvement is not sufficient yet. In addition, the improvement of the convergence and deformation of the electron beams according to the conventional methods results in a deformation in a grid image.